Field of the Invention
The invention relates to a method and a hearing aid for frequency-dependent reduction of noise in an input signal, wherein the input signal is separated into a plurality of frequency bands each having a certain bandwidth and center frequency. Wherein in each frequency band, a reduction parameter is determined from the signal component of the input signal, and wherein in each frequency band, noise in the input signal is reduced by the reduction parameter for that frequency band.
In a hearing aid, in order to improve the hearing quality for a user, the need may arise, depending on the current hearing situation for the user, to perform noise reduction on an input signal that is captured by the microphone(s) of the hearing aid. Often, for example, a user is in a hearing situation in which the existence of wideband noise impedes the actually required perception of a wanted signal. In particular in this situation, the wanted signal may have a relatively narrowband frequency spectrum and originate from a spatially clearly definable sound source, whereas the noise cannot be associated with a clearly defined spatial direction owing to superposition of a multiplicity of sound sources having different frequency spectra and different relative positions with respect to the user.
The technical challenge in a hearing aid is then to lower the level of the noise in the input signal as far as possible, thereby reducing the noise, and at the same time this lowering is meant to have the minimum possible effect on the level of the wanted signal, i.e. the aim is to improve the signal-to-noise ratio, with only limited space being available in the hearing aid itself for installing the devices for the required signal processing.
In the hearing situation described, in which wideband noise is superimposed on a narrowband wanted signal and the noise is meant to be reduced, it is usual to apply frequency-band dependent amplification or reduction coefficients to the input signal. In this case, the input signal first passes through a filter bank, which separates the input signal into a plurality of frequency bands which each have a certain bandwidth and center frequency and which partially overlap. Then a reduction coefficient is determined for each individual frequency band from the level of the signal component in the frequency band, and the level of the input signal in the individual frequency bands is lowered or raised by the respective reduction coefficients. Hence in frequency bands in which there is no wanted signal but only noise, the signal level is reduced in proportion to the wanted signal. In frequency bands in which there is a high spectral component of the wanted signal, there is less of a drop in the level or the signal level is increased more strongly. The overall effect of this is to improve the signal-to-noise ratio.
As a result of the overlap of adjacent frequency bands, it can often happen, however, that a narrowband wanted signal whose main spectral components are primarily concentrated in one specific frequency band also results in an appreciable signal level in an adjacent frequency band owing to the overlap. This affects the reduction coefficient for lowering the signal level in the adjacent frequency band. The result is that less noise reduction takes place in the adjacent frequency band concerned owing to this effect on the reduction coefficient. Hence there is a poorer signal-to-noise ratio especially in the spectral region around the wanted signal.